The present invention generally relates to methods and systems for controlling access to a bus in a network. More specifically, the invention relates to decentralized bus access control systems.
When a plurality of units, for example, processors, are coupled to a bus which, for example, can be comprised of address, data and control lines, care should be exercised to ensure that only one unit utilizes the bus at any given time. Otherwise, a conflict can arise and the bus can become tied up.
The control of access of units to the bus is referred to as arbitration. Two control protocols are currently known for this purpose. In that regard, the arbitration can be centrally controlled by an arbiter or the arbitration can be decentrally controlled by and between the processors or units.
In decentralized arbitration, there are two standard methods employed. One method is referred to as CSMA-CD (carrier sense multiple access with collision detection) wherein a processor or unit is permitted to write onto the bus whenever the bus is available. However, if another processor uses the bus at the same time, a collision occurs and then the write event for a given unit or processor must be repeated. This method is explained under the heading "CSMA/CD-Verfahren" at page 147 in Muller, Lobel, Schmidt Lexikon der Datenverarbeitung, 1992, a copy of which accompanies this application, and the teachings of which are incorporated herein by reference.
The other standard method of decentralized arbitration or access control employs a token passing scheme in which a token is passed among the processes or units connected to the bus. In such method, the only processor or unit allowed to transmit on the bus is that unit with the token. The token can be made up of a processor address and control information. This method also is explained in the same reference under the heading "Token-Verfahren" at page 667.